The outer layers of the sun are the photosphere, the chromosphere, the transition region, and the corona.
photosphere – The deepest layer of the sun that we can observe directly.
It extends from the surface visible in the center of the solar disk to about 400 km above it.
The temperature in the photosphere varies between about 6500 K below and 4000 Kelvin above (11,000 and 6700 °F, 6200 and 3700 °C).
Granulation covers most of the photosphere.
chromosphere – A layer in the Sun between about 250 miles (400 km) and 1300 miles (2100 km) above the Sun’s surface (the photosphere).
The temperature in the chromosphere varies between about 4000 Kelvin below (the so-called temperature minimum) and 8000 Kelvin above (6700 and 14,000 °F, 3700 and 7700 °C).
So this layer (and higher layers) actually gets hotter as you get farther from the sun, as opposed to the lower layers, where it gets hotter as you get closer to the center of the sun.
transition region – A very narrow (60 miles / 100 km) layer between the chromosphere and the corona where the temperature rises abruptly from about 8000 to about 500,000 Kelvin (14,000 to 900,000 °F, 7700 to 500,000 °C).
corona – The corona is the outermost layer of the sun and begins at about 1300 miles (2100 km) above the sun’s surface (the photosphere).
The temperature in the corona is 500,000 Kelvin (900,000 °F, 500,000 °C) or more, up to a few million K.
The corona cannot be seen with the naked eye except during a total solar eclipse or with the help of a coronagraph.
Corona knows no upper limit.